The present invention relates generally to illumination optical systems, and more particularly to an illumination optical system used to manufacture devices, such as single crystal plates for semiconductor wafers and glass plates for liquid crystal displays (“LCD”). The present invention is suitable, for example, for an illumination optical system for projection exposure to an object with a contact-hole line pattern or a mixture of isolated contact hole and contact-hole line in a photolithography process.
Recent demands on smaller and thinner profile electronic devices have increasingly demanded finer semiconductor devices to be mounted onto these electronic devices. In general the photolithography process is used to manufacture highly integrated devices, such as a semiconductor device, a LCD, and a thin film magnetic head. A projection exposure apparatus is an indispensable apparatus for this process to expose a pattern formed on a mask or reticle (these terms are used interchangeably in this application), onto a photoresist-applied substrate, such as a silicone wafer and a glass plate.
The following equation provides the resolution R of the projection exposure apparatus using a light-source wavelength λ and a numerical aperture (“NA”) of the projection optical system:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            
A focus range that may maintain certain imaging performance is called a depth of focus (“DOF”), which is defined in the following equation:                     DOF        =                              k            2                    ×                      λ                          NA              2                                                          (        2        )            
While Equations 1 and 2 indicate that a shorter wavelength and a larger NA are effective to finer processing, it is unfeasible since the DOF disadvantageously decreases in inverse proportion to the NA. In addition, the shorter wavelength would disadvantageously reduce transmittance of a glass material, and a larger NA makes difficult a design and manufacture of an optical system.
Accordingly, the resolution enhanced technology (“RET”) has been recently proposed which reduces the process constant k1 for fine processing. One RET is modified illumination, which is also referred to as modified illumination, multipolar illumination, or off-axis illumination. The modified illumination arranges an aperture stop with a light blocking plate on an optical axis in an optical system near an exit surface of a light integrator for forming a uniform surface light source, and introduces the exposure light oblique to a mask, as disclosed, for example, in Japanese Laid-Open Patent Application No. 5-47628 (corresponding to EP Al 589103). The modified illumination includes annular illumination and quadrupole illumination, etc. according to shapes of aperture stops.
The annular illumination provides a donut-shaped effective light source area VI on a pupil surface in a projection optical system, as shown in FIG. 24. As the light through center part in the effective light source does not contribute to imaging of a small critical dimension, a stop etc. physically shields the light from the center part to improve the entire resolving power. The quadrupole illumination provides four effective light source areas VI with a certain radius from an optical axis Sa and certain light intensity at circumferential positions, and remarkably improves both the longitudinal and lateral pattern resolving power and DOF by shielding the light on a cross area NI as well as the central part in the effective light source. Here, FIG. 24 is a plane view of an aperture stop having an annular shape, and FIG. 25 is a plane view of an aperture stop having a quadrupole shape. In general, these illuminations are effectively used because a circuit pattern, such as those for an IC and LSI, usually includes a pattern defined by longitudinal and lateral sides and rarely includes diagonal sides.
However, the above illuminations have a common problem in that they cannot vary freely illumination conditions (more specifically, an effective light source shape) and thus cannot provide high resolution to mask patterns whose sizes and arrangements change every process.
The annular illumination and multipolar illumination are means for projecting a mask pattern onto a wafer with high precision and high resolution, and the mask pattern generally is supposed to be a set of longitudinal and lateral line segments. For example, the bipolar illumination arranges an aperture stop shown in FIG. 26 at a side of an exit side of an optical integrator, which aperture stop provides two lateral effective light source areas or openings VI of predetermined light intensity around the optical axis Sa. Here, FIG. 26 is a plane view of the aperture stop having a dipole shape. The aperture stop shown in FIG. 26 is differently used so that it orientates the openings VI longitudinally in resolving a longitudinal pattern with high resolution, and laterally in resolving a lateral pattern with high resolution. A diameter of the opening VI is adjustable according to patterns' critical dimensions and scan-exposure accuracies. A turret plate 1000 arranges plural aperture stops 1100a to 1100f that provide different effective light source shapes as shown in FIG. 27, and selects and uses one of them by inserting the same into and eject the same from the optical path. FIG. 27 is a plane view of a turret plate 1000 to turn plural aperture stops 1100a to 1100f. 
Naturally, many types of aperture stops 1100a to 1100f should be prepared in order to select one of finite number of stop plates according to mask-pattern shapes. As discussed, a selection of the stop shape depends upon an addressed pattern direction in a longitudinal or lateral direction. An additional different stop shape would be required for a diagonal pattern other than the longitudinal and lateral directions, making a structure of the turret complex.
In addition, a mechanism has a problem, which rectifies errors of the process precision, assembly precision and correction precision in an apparatus that actually uses the turret plate 1000. For example, a scanning projection exposure apparatus that exposes a mask pattern onto a wafer by synchronously scanning the mask and wafer, as proposed in Japanese Laid-Open Patent Application No. 2000-164498, exhibits synchronization precision in the scan direction inferior to that in the non-scan direction, when precisions of its actual machine are measured, and thus causes different resolving power as imaging performance between the scan direction and non-scan direction. Therefore, while the annular illumination usually uses an aperture stop that has a circular effective light source area VIb as shown in FIG. 28, Japanese Laid-Open Patent Application No. 2000-164498 uses an aperture stop having an elliptical effective light source area VId as shown in FIG. 29, maintaining the optical asymmetry between the scan and non-scan directions. Here, FIG. 28 is a plane view of an aperture stop having a circular annular aperture. FIG. 29 is a plane view of an aperture stop having an elliptical annular aperture.
Although Japanese Laid-Open Patent Application No. 2000-164498 proposes to add the aperture stop having an elliptical annular shape shown in FIG. 29 to a turret plate, this reference does not refer to the multipolar illumination. In addition, the elliptical aperture stop has such a fixed shape that it cannot structurally adjust a size and distortion (ratio) of the ellipse.